Method for fabricating temperature sensitive and sputter target assemblies using reactive multilayer joining

ABSTRACT

A method for joining component bodies of material over bonding regions of large dimensions by disposing a plurality of substantially contiguous sheets of reactive composite materials between the bodies and adjacent sheets of fusible material. The contiguous sheets of the reactive composite material are operatively connected by an ignitable bridging material so that an igniting reaction in one sheet will cause an igniting reaction in the other. An application of uniform pressure and an ignition of one or more of the contiguous sheets of reactive composite material causes an exothermic thermal reaction to propagate through the bonding region, fusing any adjacent sheets of fusible material and forming a bond between the component bodies.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation, and claims priority from,U.S. patent application Ser. No. 12/029,256 filed on Feb. 11, 2008,which in turn is a divisional of U.S. Ser. No. 11/393,055 filed Mar. 30,2006 (now U.S. Pat. No. 7,354,659), which in turn is related to andclaims priority from U.S. Provisional Application Ser. No. 60/666,179filed on Mar. 30, 2005.

The present application is further a continuation of U.S. patentapplication Ser. No. 11/851,003 filed on Sep. 6, 2007, which in turn isrelated and claims priority to U.S. Provisional Application Ser. No.60/825,055 filed on Sep. 8, 2006, which is herein incorporated byreference. The '003 application is further related to and claimspriority from, U.S. Provisional Application Ser. No. 60/915,823 filed onMay 3, 2007. All of the above identified applications are incorporatedherein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The United States government has certain rights in this inventionpursuant to NSF Award DMI-034972.

BACKGROUND OF THE INVENTION

This invention relates to the joining of bodies of material over bondingregions of large dimension using reactive composite materials such asreactive multilayer foils.

Reactive composite joining, such as shown in U.S. Pat. No. 6,534,194 B2to Weihs et al and in U.S. Pat. No. 6,736,942 to Weihs et al. is aparticularly advantageous process for soldering, welding, or brazingmaterials at room temperature. The process involves sandwiching areactive composite material (RCM) between two layers of a fusiblematerial. The RCM and the fusible material are then disposed between thetwo components to be joined, and the RCM is ignited. A self-propagatingreaction is initiated within the RCM which results in a rapid rise intemperature within the RCM. The heat released by the reaction melts theadjacent fusible material layers, and upon cooling, the fusible materialbonds the two components together.

Alternatively, depending upon the composition of the two components, thelayers of fusible material are not used, and the reactive compositematerial is placed directly between the two components. Thermal energyreleased by ignition of the RCM melts material from the adjacentcomponent surfaces and consequently joins the components.

Turning to FIG. 1, an arrangement 9 for performing the process ofreactive composite joining of two components 10A and 10B is illustrated.A sheet or layer of reactive composite material 12 is disposed betweentwo sheets or layers of fusible material 14A and 14B which, in turn, aresandwiched between the mating surfaces (not visible) of the components10A and 10B. The sandwiched assembly is then pressed together, assymbolized by vise 16, and the reactive composite is ignited, as bymatch 18. The reaction propagates rapidly through the RCM 12, meltingfusible layers 14A and 14B. The melted layers cool, joining thecomponents 10A and 10B together. The RCM 12 is typically reactivemultilayer foil, and the fusible materials 14A and 14B are typicallysolders or brazes.

The process of joining the two components 10A and 10B occurs morerapidly with a reactive composite joining process than with conventionaljoining techniques such as those which utilize furnaces or torches.Thus, significant gains in productivity can be achieved. In addition,with the very localized heating associated with the reactive compositejoining process, temperature sensitive components, as well as dissimilarmaterials such as metals and ceramics, can be soldered or brazed withoutthermal damage. Fine-grained metals can be soldered or brazed togetherusing a reactive composite joining process without grain growth, andbulk amorphous materials can be welded together with only a localexcursion from room temperature, producing a high strength bond whileminimizing crystallization.

The reactive composite materials 12 used in reactive composite joiningprocess are typically nanostructured materials such as described in U.S.Pat. No. 6,534,194 B2 Weihs et al. The reactive composite materials 12are typically fabricated by vapor depositing hundreds of nano-scalelayers which alternate between elements having large, negative heats ofmixing, such as nickel and aluminum. Recent developments have shown thatit is possible to carefully control both the heat of the reaction aswell as the reaction velocity by varying the thicknesses of thealternating layers. It has also been shown that the heats of reactioncan be controlled by modifying the foil composition, or bylow-temperature annealing of the reactive multi-layers after theirfabrication. It is further known that alternative methods forfabricating nanostructured reactive multilayers include mechanicalprocessing.

Two key advantages achieved by the use of reactive composite materialsfor joining components are speed and the localization of heat to thejoint area. The increased speed and localization are advantageous overconventional soldering or brazing methods, particularly for applicationsinvolving temperature-sensitive components or components with a largedifference in coefficient of thermal expansion, such as occur inmetal/ceramic bonding. In conventional welding or brazing,temperature-sensitive components can be destroyed or damaged during theprocess. Residual thermal stress in the components may necessitatecostly and time-consuming operations, such as subsequent anneals or heattreatments. In contrast, joining with reactive composites subjects thecomponents to little heat and produces only a very local rise intemperature. Generally, only the adjacent fusible layers and theadjoining surfaces of the components are heated substantially. Thus, therisk of thermal damage to the components is minimized. In addition,reactive composite joining is fast and results in cost-effective,strong, and thermally conductive joints.

In one aspect, the invention relates to the joining of large areaassemblies especially the joining of temperature sensitive materials.While conventional reactive composite joining works well in joiningcomponents over lengths less than about four inches and areas less thanabout 16 square inches, joining over larger lengths and areas presentsparticular challenges. It has been observed that for optimal joining itis advantageous that the surfaces to be joined be heated as uniformly,and as simultaneously, as possible. When the lengths and areas becomelarger, it is increasingly difficult to maintain the desired reactionsimultaneity and uniformity from a single ignition point. In addition,larger joining region dimensions can exceed those of easily fabricatedRCM's, requiring multiple pieces of reactive foil to cover the jointsurface area. Even though the joining reaction spreads rapidly throughthe RCM, not every part of a large surface area joint area may be moltenat the same time, possibly resulting in poor bonding between thecomponents. Moreover, increasing the surface area to be joined presentsincreasingly stringent requirements for the uniform application ofpressure to the components during the joining process.

In another aspect, the invention relates to the manufacture of targetsfor use in physical vapor deposition processes, and in particular, to anovel method of bonding metal or ceramic tiles or plates to metalbacking plates for use as targets in physical vapor deposition processes(sputtering). Such targets are typically large area assemblies oftemperature sensitive materials.

Differences in the coefficients of thermal expansion (CTE) betweensputtering target materials and backing plate materials limit the use ofconventional soldering processes. In such conventional processing, theentire bond assembly is heated above the melting temperature of thesolder. On cooling, the excessive contraction of the component with thehigher CTE relative to the component with the lower CTE results insevere residual stresses within the bond and in the components. This isgenerally true for all ceramic targets bonded to metal backing plates.The net result is that good quality bonds are limited to very smallareas or else large area bonds are of very low quality, characterized bydebonding, cracking and warping of the target and backing platecomponents. Indium solder or elastomer bonds are often used to mitigatethese problems. Although indium is a very compliant material, it has lowstrength (tensile strength of 2 MPa) and has a very low meltingtemperature (157° C.). The resulting indium bonds are similarly weak andare unable to tolerate even moderate temperatures in service. Even ifindium solder is used, locked in residual stresses during conventionalbonding lead to poor bond quality and cracking of the ceramic targetoften results during service. Elastomer bonds on the other hand havehigher strengths, but suffer from very low electrical and thermalconductivities and outgassing issues during service.

Bonding with reactive multilayer foil is a new joining technology thatenables soldering without significantly heating the components beingbonded. The reactive multilayer foils are magnetron sputtered andconsist of thousands of alternating nanoscale layers, such asalternating layers of Ni and Al. The layers react exothermically whenatomic diffusion between the layers is initiated by an external energypulse, and release a rapid burst of heat in a self-propagating reaction.If the foils are sandwiched between layers of solder, the heat releasedby the foils can be harnessed to melt these layers. By controlling theproperties of the foils the exact amount of heat released by the foilscan be tuned to ensure there is sufficient heat to melt the solderlayers, but at the same time the bulk of the components will be at orclose to room temperature. The components therefore do not undergo anysignificant expansion or contraction during bonding despite differencesin CTE. Bonding with reactive multilayer foil is thus a room temperaturemethod that enables the formation of low stress, high quality metallicbonds between materials with dissimilar CTE's.

Accordingly, it would be advantageous to provide a reactive compositejoining process for use in joining components over surface areas whichare larger than the size of a single sheet of reactive compositematerial, and which result in a strong and relatively uniform bondbetween the component materials.

BRIEF SUMMARY OF THE INVENTION

Briefly stated, the present invention provides a method for joiningbodies of component material over regions of large dimensions bydisposing a plurality of substantially contiguous RCM sheets between thecomponent material bodies. Each of the substantially contiguous RCMsheets is coupled to at least one adjacent RCM sheet by a bridgingmaterial capable of transferring an energetic reaction from one sheet toanother. An ignition reaction is initiated in one or more of the RCMsheets and enabled to spread through all remaining sheets via thebridging material, resulting in rapid localized heating of materialsadjacent the sheets, which form a bond between the bodies of componentmaterial upon cooling.

In an embodiment of the present invention, a plurality of substantiallycontiguous RCM sheets disposed between component material bodies to bejoined over a region of large dimension are coupled together by abridging material. The bridging material may be in the form of areactive foil, wire, layer, powder, or other material which is capableof conveying an ignition reaction from one sheet to another, eitherdirectly or by thermal conduction. The bridging material is reactive inresponse to an ignition of a first RCM sheet to ignite a second RCMsheet.

In an alternate embodiment of the present invention, a plurality ofsubstantially contiguous RCM sheets disposed between component materialbodies to be joined over a region of large dimension are coupledtogether by structural support tabs of fusible material to enable easyassembly, transport, and positioning of the multiple RCM sheets betweenthe component bodies to be joined.

In a variation of the present invention, a plurality of substantiallycontiguous RCM sheets are disposed between component material bodies tobe joined over a region of large dimensions, directly adjacent surfacesof the component material bodies to be joined.

In an alternate embodiment of the present invention, a plurality ofsubstantially contiguous RCM sheets are disposed between componentmaterial bodies to be joined over a region of large dimensions. Sheetsof fusible material such as solder or braze are disposed in proximity tothe RCM sheets and to the component material bodies. The fusiblematerial sheets can overlie, underlie, or sandwich the sheets ofreactive composite materials. The fusible material sheets can becontinuous across the boundaries of the contiguous RCM sheets, and mayoptionally function as connecting material to hold RCM sheets together.

A method of the present invention for joining bodies of componentmaterial over regions of large dimension disposes at least one RCM sheetbetween the component material bodies. An ignition reaction is initiatedat a plurality of ignition points disposed about the RCM sheet,resulting in rapid localized heating of materials adjacent the sheetswhich form a bond between the bodies of component material upon cooling.

One application of the invention provides a novel method of bondingmetal or ceramic tiles or plates to metal backing plates for use astargets in physical vapor deposition processes (sputtering). In oneembodiment, the method utilizes reactive multilayer foil to heat theinterface between the plates above the melting point of solder or brazelayers pre-applied to the plates, allowing the solder or braze to fusetogether and to the foil and join the plates. This method permitssoldering or brazing of materials with large differences in coefficientof thermal expansion due to minimal heating of the plates.

A variation of the method of the present invention for joining bodies ofcomponent material over regions of large dimension disposes at least oneRCM sheet between the component material bodies. At least one spacerplate is positioned between an external pressure source and thecomponent bodies. Pressure is applied to the arrangement from theexternal pressure source, urging the component bodies towards each otherto control the formation of a bond between the component bodiesfollowing initiation of an ignition reaction in the RCM sheets. Theignition reaction within the RCM sheets results in rapid localizedheating of materials adjacent the sheets, which form a bond between thebodies of component material upon cooling.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The advantages, nature and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings. In the drawings:

FIG. 1 schematically illustrates a prior art arrangement for performingconventional reactive composite joining of two components;

FIG. 2 is a block diagram representing the steps involved in joining twobodies by a large dimension joint in accordance with the invention;

FIG. 3 is a top plan view of a plurality of contiguous reactivecomposite material sheets operatively connected by structural supporttabs and ignition bridges within the bonding region for the formation ofa large area bond between two component bodies;

FIG. 4 is a top plan view of a plurality of contiguous reactivecomposite material sheets operatively connected by structural supporttabs within the bonding region and ignition bridges external to thebonding region, for the formation of a large area bond between twocomponent bodies;

FIG. 5 schematically illustrates several contiguous reactive compositematerial sheets operatively connected by ignition bridges external tothe bonding region for the propagation of an ignition reaction fromsheet to sheet;

FIG. 6 is a cross sectional view of a pair of contiguous reactivecomposite material sheets sandwiched between two layers of a fusiblematerial;

FIG. 7 is a block diagram representing an arrangement of components andlayers for practicing a joining method of the present invention;

FIG. 8 illustrates an exemplary arrangement for simultaneously ignitinga plurality of reactive composite material sheets during formation of abonding joint in accordance with a method of the present invention;

FIG. 9 is a top-plan acoustic image of a large dimension joint formed inaccordance with a method of the present invention;

FIG. 10 is a top-plan acoustic image of a large dimension joint formedin accordance with an a method of the present invention, illustratingedge voids;

FIG. 11 is a top-plan acoustic image of a large dimension joint formedin accordance with an optimized loading bonding method of the presentinvention;

FIG. 12 illustrates an exemplary arrangement of contiguous reactivecomposite material sheets, fusible material support tabs, ignitionbridges, and ignition points;

FIG. 13 is a top-plan acoustic image of a large dimension jointresulting from the arrangement illustrated in FIG. 12;

FIG. 14 illustrates an exemplary arrangement of contiguous reactivecomposite material sheets, fusible material support tabs, ignitionbridges, and ignition points;

FIG. 15 is a top-plan acoustic image of a large dimension jointresulting from the arrangement illustrated in FIG. 14;

FIG. 16 is a block diagram representing a first exemplary arrangement ofcomponents and layers for practicing a joining method of the presentinvention; and

FIG. 17 is a block diagram representing a second exemplary arrangementof components and layers for practicing a joining method of the presentinvention.

FIG. 18 is a cross-sectional schematic showing the process of bonding asputtering target to a backing plate using reactive multilayer foil.

FIG. 19 is a cross-sectional temperature profile captured at the momentof solder solidification on cooling during bonding with reactivemultilayer foil. This profile was generated by finite differencemodeling of thermal transport and used as input for subsequent FEM.

FIG. 20 is a computed (FEM) residual von Mises stress after bonding B₄Cto CuCr using 96.5Sn3.5Ag solder. Cross-sectional view shown with halflength (3 inch) symmetry. The top plot is for a conventional bondingoperation and the bottom plot is for a bonding operation using reactivemultilayer foil as a localized heat source.

FIG. 21 is a cross section of a bond between two brass discs usingreactive multilayer foil to melt 63Sn37Pb solder. The molten solderthickness during bonding can be observed from the refined grain size ofthe solder adjacent to the reactive multilayer foil that is in thecenter of the bond. The dark areas in the solder represent a secondphase.

FIG. 22 is an ultrasonic scan of the bond line of a 12 in×12 in titaniumalloy plate bonded to an aluminum alloy backing plate formed by melting96.5Sn3.5Ag solder using reactive multilayer foil. Measure bond coverage>99%.

FIG. 23 illustrates photographs of two used B₄C sputtering targets, 25in×6 in, bonded to CuCr backing plates. The target on the left wasbonded by conventional reflow of indium solder and cracked after firstuse (shown by arrows). The target on the right was bonded by thelocalized melting of 95.5Sn3.5Ag solder using reactive multilayer foiland remained intact and fully bonded after running at twice the power ofthe target on the left for over 200 hours.

FIG. 24 illustrates photographs of ITO targets after sputtering trials.

FIG. 25 illustrates photographs of alumina targets after sputteringtrials.

FIG. 26 illustrates photographs of boron carbide targets aftersputtering use. Cracks in the conventional ln bonded target areindicated by arrows.

Corresponding reference numerals indicate corresponding parts throughoutthe several figures of the drawings. It is to be understood that thedrawings are for illustrating the concepts of the invention and are notto scale.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description illustrates the invention by way ofexample and not by way of limitation. The description enables oneskilled in the art to make and use the invention, and describes severalembodiments, adaptations, variations, alternatives, and uses of theinvention, including what is presently believed to be the best mode ofcarrying out the invention.

As used herein, the phrase “large dimension” is used to describe a jointor bonding region and is understood to mean a joint or bonding regionwhich has an area or length which exceeds the area or length of a singlesheet of reactive composite material utilized in the joining processes,which is sufficiently large enough that a single propagation wave frontfrom an ignition reaction within a sheet of reactive composite materialfails to achieve desired bond characteristics throughout the bondingregion, or which exhibits a loading variation between the center and theedges of the joint or bonding region. For example, an area of at least16.0 sq. inches or a length of at least 4.0 inches is considered to be alarge dimension when utilizing a sheet of reactive composite materialhaving an area of less than 16.0 sq. inches and a longest dimension ofless than 4.0 inches.

As used herein, the phrase “reactive composite material” or “RCM” isunderstood by those of ordinary skill in the art to refer to structures,such as reactive multilayer foils, comprising two or more phases ofmaterials spaced in a controlled fashion such that, upon appropriateexcitation or exothermic reaction initiation, the materials undergo anexothermic chemical reaction which spreads throughout the compositematerial structure. These exothermic reactions may be initiated byelectrical resistance heating, inductive heating, laser pulses,microwave energy, or ultrasonic agitation of the reactive compositematerial at one or more ignition points.

Referring to the drawings, FIG. 2 illustrates a generalized flow diagramof the steps involved in joining together two component bodies 10A and10B over a joint or bonding region having a large dimension (large areaor large length), using at least two contiguous sheets 12 of reactivecomposite material. Initially, as shown in block A, two component bodies10A and 10B to be joined over substantially conforming large dimensionmating surfaces are provided. The two component bodies 10A and 10B maybe coated in advance with one or more layers of a fusible material 14Aand 14B, such as a solder or braze alloy, or one or more sheets of thefusible material 14A and 14B may be placed between the component bodies10A and 10B. The component bodies 10A and 10B may comprise the same typeof materials, such as brass, or may be of different types of materials,such as nickel and brass, aluminum and titanium, boron carbide andsteel, boron carbide and copper, silicon carbide and aluminum, and atungsten-titanium alloy and a copper-chromium alloy.

Next, as shown in Block B, two or more sheets 12 of reactive compositematerial in a substantially contiguous arrangement are disposed betweenthe mating surfaces of the two component bodies 10A and 10B. As usedherein, the term “contiguous” is understood by those of ordinary skillin the art to mean that any adjacent edges of the sheets 12 of reactivecomposite material are arranged as close together as necessary to form asubstantially void-free bond and at least sufficiently close togethersuch that adjacent sheets 12 of reactive composite material can beoperatively connected together into a single assembly. Contiguous RCMsheets do not need to be in physical contact with each other.

To operatively connect adjacent RCM sheets 12, a number of structurallysupporting bridges or tabs 20 are formed between the sheets 12 (as shownin FIGS. 3 and 4). The bridges or tabs may be formed from either afusible material 20 which will form part of the bond between thecomponent bodies 10A and 10B, or may be formed from reactive material 22which is capable of conveying an ignition reaction between adjacentsheets of RCM. Using the bridges or tabs 20, 22, two or more adjacentRCM sheets 12 are secured together in an assembly 24 in such a way as tomaintain the relative positions to each other during assembly,transport, and positioning between the matching surfaces of thecomponent bodies 10A and 10B in the large dimension bonding region.

A structural support bridge or tab 20 can be in any one of several formsto secure contiguous RCM sheets 12 together in the assembly 24. In oneexemplary embodiment, the structural support bridges or tabs 20 are inthe form of a soft metal or fusible material sheet, for instance indium,which is cold-pressed or rolled onto the RCM sheets 12.

An ignition bridge or tab 22 formed from a reactive material ispreferably selected such that it will either ignite or conduct thermalenergy between the adjacent sheets 12 to enable a reaction initiated ina first sheet 12A to continue via the bridge or tab to the adjacentsheet 12B. The configuration of an ignition bridge or tab 22 can be inany one of several forms to assist propagation of reaction betweencontiguous RCM sheets 12. For example, the ignition bridge or tab 22 canbe in the form of a reactive multilayer foil, similar or identical tothat used for the RCM sheets 12, or a thin wire that contains regions orlayers of materials with a large negative heat of mixing. Theseconfigurations of the ignition bridges or tabs 22 can be attached to oneor both contiguous sheets 12 with a small amount of glue or with a smallpiece of fusible solder. In addition to conveying an initiated reaction,ignition bridges or tabs 22 may be structural in nature, i.e. providingstructural support to an arrangement of sheets 12 of RCM, or may benon-structurally supporting in nature, For example, a non-structurallysupporting ignitable bridge 22 can be in the form of a loose or compactpowder mixture of materials with a large negative heat of mixing.

Advantageously, the various forms of both bridges and tabs 20, 22 aresmall in comparison to the size of the RCM sheets 12, and do notinterfere with the flow of any fusible material present in the bondingregion, or with the flatness of the component body mating surfacesduring the joining process.

Turning to FIG. 3, an exemplary arrangement of a plurality of contiguousRCM sheets 12 are shown arranged and connected by solder assembly tabs20 and ignitable bridges 22 to form a reactive composite material sheetassembly 24 covering a large area bonding region 26 between twocomponent bodies (not shown). In the arrangement shown in FIG. 3, boththe solder assembly tabs 20 and the ignitable bridges 22 are containedwithin the large area bonding region 26. The arrows arranged about theperiphery of the assembly 24 indicate a plurality of ignition orreaction initiation points associated with the assembly 24.

FIG. 4 illustrates a second exemplary arrangement of a plurality ofcontiguous RCM sheets 12 arranged and connected by solder assembly tabs20 and ignitable bridges 22 to form a reactive composite material sheetassembly 24 covering a large area bonding region 26 between twocomponent bodies (not shown). In the arrangement shown in FIG. 4, thesolder assembly tabs 20 are contained within the large area bondingregion 26, while the ignitable bridges 22 are disposed outside the largearea bonding region 26. By disposing the ignitable bridges 22 outside ofthe large bonding region 26, the ignitable bridges 22 may be attached tothe sheets 12 of the assembly 24 with tape or other means that would notbe possible within the large area bonding region. The bridges may thusbe used for securing the sheets 12 within the assembly 24, as well asfor ignition of the bonding reactions, as is indicated by the arrowsarranged about the periphery of the assembly 24 to indicate a pluralityof ignition or reaction initiation points.

Within a large area bonding region 26, the solder tabs 20 may be securedto the sheets 12 of the assembly 24 by pressing or with a minimal amountof glue. If it is undesirable to use a solder material for the tabs 20which differs from the solder material used as a fusible material withinthe joint, due to concerns about alloying, small tabs of the desiredsolder could be glued to the reactive sheets, preferably minimizing theamount of glue.

FIG. 5 illustrates a third exemplary arrangement of a plurality ofcontiguous RCM sheets 12 arranged and connected by solder assembly tabs20 and ignitable bridges 22 to form an RCM sheet assembly 24 covering alarge linear dimensioned bonding region 30 between two component bodies(not shown). The ignitable bridges 22 may be attached to the reactivesheets 12 with glue or small pieces of solder material if they aredisposed inside a joint region or with adhesive tape if they aredisposed outside a joint region.

Those of ordinary skill in the art will recognize that the number of RCMsheets 12 comprising the various assemblies 24 shown in FIGS. 3, 4, and5 may be varied depending upon the size and configuration of the largearea bonding region 26 or large linear dimensioned bonding region 30.Preferably, RCM sheets 12 are arranged within the assemblies 24 suchthat the gaps G between adjacent sheets 12 are as far to the interior ofthe bonding region as possible, particularly gaps G which are parallelto the edges of the bonding region. Similarly, it will be recognizedthat the placement and number of tabs 20 and ignition bridges 22 may bevaried depending upon the particular application and geometry of theassembly 24, provided that the assembly 24 is secured in a stableconfiguration during placement in the bonding region, and that reactionscan propagate between the sheets 12 of the assembly 24 in a generallyrapid and uniform manner.

In lieu of assembly tabs 20, an assembly 24 of two or more RCM sheets 12with ignition bridges 22 may be packaged as shown in FIG. 6 betweenlayers 32A and 32B of a fusible material, such as a solder or a braze.Such packaging allows the fusible material layers 32A and 32B and theassembly 24 of RCM sheets 12 to be handled as a unit, aiding inplacement within a bonding region. The RCM sheets 12 may be bonded tothe fusible material layers 32A and 32B by rolling, pressing, or othersuitable means to ensure that the packaging remains structurally secure.

Once the assembly 24 is formed, with or without fusible layers 32A and32B, it is disposed within the bonding region 26 between the components10A and 10B to be joined. As shown in Block C of FIG. 2, the components10A and 10B to be joined are pressed together to provide a generallyuniform pressure over the bonding region 26. A variety of devices andtechniques may be utilized to achieve the generally uniform pressurebetween components 10A and 10B over the bonding region 26, for example,hydraulic or mechanical presses. As is shown in FIG. 7, the components10A and 10B may be disposed with suitable spacers 34 between a pair ofpress platens 36A and 36B. The selection of the spacers 34, and theireffect on the resulting bond formed within the bonding region 26 aredescribed below in further detail. An optional compliant layer 38, suchas a rubber sheet, may be placed in the component arrangement betweenone component 10A or 10B and an adjacent press platen to accommodate anyimperfections on the outside surfaces of the components 10A and 10B andthe surfaces of the press platens 36A and 36B during the joiningprocess. Pressure is exerted on the press platens 36A and 36B by anysuitable means, such as by a hydraulic press with automatic pressurecontrol.

The final step, shown in Block D of FIG. 2, is to ignite the RCM sheets12 comprising the assembly 24. When bonding large areas, it isadvantageous to symmetrically ignite the RCM sheets 12 at multiplepoints about the peripheral edge of the assembly 24, such as illustratedby the ignition point arrows in FIGS. 3 and 4. Interior sheets 12 whichdo not have edges outside the bonding region 26 are then ignited by thepropagation of the reaction across the ignition bridges 22 within theassembly 24 as discussed previously. Simultaneous ignition of the sheets12 in an assembly 24 can be conveniently effected by simultaneousapplication of an electrical impulse, such as shown in FIG. 8, or bylaser impulse, induction, microwave radiation, or ultrasonic energy.

Those of ordinary skill in the art will recognize that a variety ofdevices which are capable of simultaneous delivery of ignition energy tothe ignition points may be used. For example, an electrical circuitconsisting of a capacitor and a switch associated with each ignitionpoint may be employed. All the switches are controlled by a masterswitch, such that the capacitors charge and discharge simultaneously. Anelectrical pulse travels from the capacitors, through the switches tothe ignition points on the RCM sheets 12, and to an electrical groundthrough the press platens 36A and 36B, igniting the sheets 12 within theassembly 24 and ultimately forming the bond between components 10A and10B. Alternatively, a single large capacitor and switch may be connectedto all the ignition points in parallel, such that energy is dischargedto all ignition points about the assembly 24 simultaneously from thecapacitor to ignite each sheet 12.

During the bonding process, it is known that non-uniform loaddistribution between the component bodies 10A and 10B will result inpoor quality bonds with the presence of air gaps (voids) following theignition of the sheets 12 within the assembly 24. Uneven loaddistribution typically results when the press platens 36A and 36B of theloading mechanism are significantly oversized or undersized compared tothe size of the bonding region 26. This problem may be exacerbated whenone or both of the components 10A and 10B to be joined are relativelythin. In the case where the press platens 36A and 36B are oversizedrelative to the size of the bonding region 26, the resulting pressurenear the peripheral edges of the bonding region 26 is greater than thepressure near the center of the bonding region 26, and thus voids mayform near the center of the bonding region 26. This is illustrated bythe white regions visible near the center of the top-plan ultrasonicacoustic image or C-scan of a bonding region 26 shown in FIG. 9.

Conversely, in the case where the press platens 36A and 36B areundersized relative to the bonding region 26, the pressure near thecenter of the bonding region is greater than the pressure near theperipheral edges of the bonding region 26, and thus voids may appearabout the peripheral edge as is shown by the white regions visible aboutthe peripheral edges of the top-plan ultrasonic acoustic image or C-scanof a bonding region 26 shown in FIG. 10.

In order to distribute the load from the press platens 36A and 36B in auniform manner to the bonding region 26, one or more spacer plates 34sized to match the bonding region 26 are placed between the components10A, 10B, and the platen or platens 36A, 36B. The ideal thickness forthe spacer plate or plates 34 may be determined by a sequential process,in which a test bond is initially formed without the use of any spacerplate or plates 34. The resulting bond between components 10A and 10B isevaluated to identify the presence of voids. For applications where thepress platens 36A and 36B are larger than the bonding region 26, thebond quality may be characterized by a ratio of voided area in thecenter quarter of the bonding region 26 to the total area of the bondingregion. To reduce the voided area, spacer plates 34 of increasinglygreater thickness are employed in additional bonding test proceduresbetween components 10A and 10B until the desired ratio of voided areasto bonding region area is achieved for a bonding procedure. Preferably,the thickness of the spacer plates 34 is doubled between each bondingtest procedure until the desired ratio is achieved.

The procedure may be modified for large area joining applications wherenone or only a limited number of edge voids can be tolerated. For theseapplications the percentage of edge voids, defined as the ratio ofvoided area in the outer quarter of the bonding region 26 to the totaljoining area, may be tracked as described above. If the process ofdoubling the spacer plate thickness results in an acceptable percentageof center voids and no edge voids, then the optimal spacer platethickness has been derived. If on the other hand, the process results inan acceptable percentage of center voids, but some percentage of edgevoids are detected, then the spacer plate thickness should be reduced tothe average thickness of the present and previous spacer platethicknesses. This process is repeated until a spacer plate 34 having adetermined thickness results in the minimum amount of center voids andthe desired amount of edge voids. This is illustrated by the small whiteregion near the center and the general lack of any white regions visiblenear the peripheral edges of the top-plan ultrasonic acoustic image orC-scan of a bonding region 26 shown in FIG. 11.

For applications where the press platens 36A and 36B are undersizedrelative to the bonding region 26, the bond quality may be characterizedby a ratio of voided area in the outer quarter of the bonding region 26to the total area of the bonding region. To reduce the voided area,spacer plates 34 of increasingly greater thickness are employed inadditional bonding test procedures between components 10A and 10B untilthe desired ratio of voided areas to bonding region area is achieved forthe bonding procedure. Preferably, the thickness of the spacer plates 34is doubled between each bonding test procedure until the desired ratiois achieved.

The methods of the present invention for joining component bodies 10Aand 10B over a large dimension bonding region 26 are further illustratedby the following six examples.

Example 1

In this example, reactive or ignition bridges 22 and assembly tabs 20were disposed on an assembly 24 inside the peripheral edges of a bondingregion 26 as is illustrated in FIG. 3. As shown in the generalarrangement of FIG. 7, the various components were assembled betweenpress platens 36A and 36B, with the bonding region 26 to be formedbetween a nickel disk component 10A (0.2 in. thick) and a brass diskcomponent 10B (0.6 in. thick). The bonding region 26 was circular, withan outer diameter of 17.7 in. and an area of 246 sq. inches. Layers offusible material 32A and 32B, such as tin-lead solder, were pre-appliedto the nickel and brass bodies 10A and 10B. To provide coverage for thebonding region 26, a total of sixteen RCM sheets 12 (Ni—Al, 80 μm thick,reaction velocity 7 m/s) were pre-assembled as an assembly 24 as shownin FIG. 3, by pressing a total of twelve indium solder tabs 20 acrossthe gaps G. To ensure reaction propagation across the gaps G, sixreactive foil ignition bridges 22 were attached across gaps G within thebonding region 26. Small pieces of indium solder were additionally usedto affix the reactive foil ignition bridges 22 to the RCM sheets 12.

The brass disk 10B was placed on a flat surface with the pre-appliedlayer of tin-lead solder 32B facing upwards. The portions of theassembly 24 were positioned adjacent to each other with a minimumseparation gap G on top of the brass disk 10B so that they completelycovered the bond region 26. The nickel disk 10A was placed above thereactive multilayer foil with the pre-applied layer of tin-lead solder32A facing down, in contact with the RCM sheets 12 (Ni—Al, 80 μm thick,reaction velocity 7 m/s) in the assembly 24. An aluminum spacer plate 340.75 inches thick, with a diameter of 17.7 inches, was positioned aboveand aligned with the nickel disk 10A. The spacer thickness waspreviously determined using the process described above, by makingseveral joints with different sized spacer plates. A thin layer of hardrubber 38, with a matching surface area, was placed above the aluminumspacer plate 34 to accommodate any imperfections on the outside surfacesof the brass and nickel disks 10A and 10B and the surfaces of theplatens of the press 36A, 36B used to apply a load during joining. Theentire arrangement was transferred to a hydraulic press, where a load of107,000 lbs was applied to the arrangement. The sheets 12 of theassembly 24 were then ignited electrically, simultaneously at twelveignition points around the circumference identified by the arrows inFIG. 3, resulting in the bonding of the component bodies 10A and 10B toeach other.

Example 2

In this example, assembly tabs 20 were disposed on an assembly 24 insidethe peripheral edges of a bonding region 26, while the reactive orignition bridges 22 were disposed outside the peripheral edges of thebonding region 26, as is illustrated in FIG. 4. As shown in the generalarrangement of FIG. 7, the various components were assembled betweenpress platens 36A and 36B, with the bonding region 26 to be formedbetween a nickel disk component 10A (0.2 in. thick) and a brass diskcomponent 10B (0.6 in. thick). The bonding region 26 was circular, withan outer diameter of 17.7 in. and an area of 246 sq. inches. As withExample 1, above, layers of fusible material 32A and 32B, such astin-lead solder, were pre-applied to the nickel and brass bodies 10A and10B. To provide coverage for the bonding region 26, a total of eight RCMsheets 12 were pre-assembled as an assembly 24 as shown in FIG. 4, bypressing a total of three indium solder tabs 20 across the gaps G. Toensure reaction propagation across the gaps G, eight reactive foilignition bridges 22 were attached across gaps G outside the bondingregion 26. Small pieces of high temperature tape (Kapton®) were used toprovide adhesion between the ignition bridges 22 and the RCM sheets 12,and to further serve the purpose of providing structural support to theassembly 24.

Next, the brass disk 10B was placed on a flat surface with thepre-applied layer of tin-lead solder 32B facing upwards. The portions ofthe assembly 24 were positioned adjacent to each other with a minimumseparation gap on top of the brass disk 10B so that they completelycovered the bond region 26. The nickel disk 10A was placed above thereactive multilayer foil with the pre-applied layer of tin-lead solder32A facing down, in contact with the RCM sheets 12 in the assembly 24.An aluminum spacer plate 34 0.75 inches thick, with a diameter of 17.7inches, was positioned above and aligned with the nickel disk 10A. Thespacer thickness was previously determined using the process describedabove, by making several joints with different sized spacer plates. Athin layer of hard rubber 38, with matching surface area, was placedabove the aluminum spacer plate 34 to accommodate any imperfections onthe outside surfaces of the brass and nickel disks 10A and 10B, and thesurfaces of the platens of the press 36A, 36B used to apply a loadduring joining. The entire arrangement was transferred to a hydraulicpress, where a load of 107,000 lbs was applied to the arrangement. Thesheets 12 of the assembly 24 were then ignited electrically,simultaneously at sixteen ignition points around the circumferenceidentified by the arrows in FIG. 4, resulting in the bonding of thecomponent bodies 10A and 10B to each other.

Example 3

In this example, assembly tabs 20 and ignition bridges 22 were disposedon an assembly 24, both inside and outside of the peripheral edges of abonding region 26, as is illustrated in FIG. 12. Component bodies 10Aand 10B consisting of a 0.3″ thick copper alloy disk (10A) and a 0.5″thick copper alloy disk (10B) were arranged with the assembly 24 in thebonding region 26, according to the general arrangement shown in FIG. 7.The bonding region 26 was circular, with a diameter of 13 inches and anarea of 133 sq. inches. Tin-lead solder was used as the fusible materiallayers 32A and 32B. Nine RCM sheets 12 were pre-assembled in theassembly arrangement 24 shown in FIG. 12 by pressing four indium soldertabs 20 across sheet gaps G. To ensure reaction propagation across thesheet gaps G, ten reactive ignition bridges 22 were attached at criticalboundaries, two within the bonding region 26 and eight outside thebonding region 26. The various components were arranged as shown in FIG.7 with the 0.5″ copper disk 10B at the bottom, but with no optionalspacer plate 34 below it, then the assembly 24 was positioned within thebonding region 26, and then the 0.3″ copper disk 10A placed on top. Analuminum spacer plate 34 was positioned above, and aligned with, the0.3″ thick copper disk 10A. A thin layer of stiff foam, with a matchingsurface area dimension served as the compliant layer 78. The entirearrangement was transferred to a hydraulic press and a load of 57,850lbs was applied to the assembly 24 by the press. The RCM sheets 12 wereignited electrically, simultaneously at twelve points evenly spacedaround the circumference, as indicated by the arrows in FIG. 12, toinitiate the bond forming reaction.

The resulting joined assembly was ultrasonically (acoustically) scannedto determine the quality of the bond. A representative acoustic scan isshown in FIG. 13, with areas of poor bond quality including trapped air,know as voids, displayed as bright white regions adjacent the peripheraledge of the bonding region 26. The void content, measured as apercentage of the total bond area, is less than 1%, indicating a highquality bond. Dark lines in FIG. 13 indicate cracks or gaps betweenindividual sheets of reactive composite material that have been filledin by molten solder during the joining process. The non-straight darklines are due to cracking of individual sheets of the reactive compositematerial which occurs during joining due to volume contraction of thesheets as they react. The filled gaps between individual pieces ofreactive multilayer foil are straight lines and reveal the pattern ofindividual sheets of the reactive composite material that werepre-assembled into the assembly 24 prior to joining.

Example 4

In this example, an assembly 24 of RCM sheets 12 is arranged withassembly tabs 20 disposed within a square bonding region 26, and withignition bridges 22 outside of the peripheral edges of the squarebonding region 26, as is illustrated in FIG. 14. The components 10A and10B to be bonded consist of a square plate 10A of aluminum 0.5″ thickand a square plate 10B of titanium-aluminum-vanadium alloy 0.5″ thick.The bonding region 26 defines a square with sides 12 inches long and anarea of 144 sq. inches. A tin-silver solder was used to provide fusiblelayers 32A and 32B on the two component bodies 10A, 10B. Six equal sizedRCM sheets 12 were pre-assembled in the pattern shown in FIG. 14 bypressing two indium solder tabs 20 across gaps G between the sheets 12.To ensure reaction propagation across the gaps G, six reactivemultilayer foil ignition bridges 22 were attached at critical boundariesoutside the bonding region 26. As shown in the general arrangement ofFIG. 7, a square spacer plate 34 of aluminum, 0.5 inches thick with 12inch sides was placed on a flat surface. The titanium alloy componentbody 10B, reactive multilayer foil assembly 24, and aluminum componentbody 10A were then placed on top of the spacer plate 34. A second squarealuminum spacer plate 34 of the same dimension as the first spacerplate, was positioned above, and aligned with, the aluminum componentbody 10A. A thin layer of hard rubber disposed above the second spacerplate 34 served as a compliant layer 38. The entire assembledarrangement was transferred to a hydraulic press and a load of 62,640lbs was applied to the assembled arrangement by the press platens 36Aand 36B. The RCM sheets 12 were ignited electrically, simultaneously atten points evenly spaced around the circumference, indicated by arrowsin FIG. 14, resulting in a bonding reaction between the component bodies10A and 10B.

The resulting joint between the component bodies 10A and 10B wasultrasonically scanned to determine the quality of the bond. An acousticscan is shown in FIG. 15. The void content, measured as a percentage ofthe total bond area, is less than 1%, indicating a high quality bond.The dark horizontal and vertical lines in FIG. 15 indicate gaps betweenindividual RCM sheets 12 that have been filled by molten solder duringthe joining process. Thus from the ultrasonic C-scan it can clearly beobserved that six sheets of reactive composite material effectivelyjoined the two component bodies 10A and 10B.

Example 5

In this example, an assembly 24 of RCM sheets 12 was utilized tosimultaneously join a set of discrete component tiles 40A-40F to asingle base component body 42, as shown schematically in FIG. 16. Eachof the discrete component tiles 40A-40F is composed of boron carbide(B₄C), and has dimensions of 6.25 inches long by 6 inches wide, by 0.25inches thick. The single base component body 42 comprises a copper platehaving overall dimensions of 26.25 inches long by 7.25 inches wide by0.31 inches thick. The bonding region 26 has a rectangularconfiguration, with a length of 25 inches, a width of 6 inches, and anarea of 150 sq. inches. Layers of tin-silver solder pre-applied to thecopper plate 42 and to each boron carbide tile 40A-40F act as fusiblematerial layers 32A and 32B. Six RCM sheets 12 were pre-assembled intoan assembly 24 by taping ten reactive multilayer foil ignition bridges22 across gaps between the sheets 12 outside of the bonding region 26,as shown in FIG. 5, increasing the probability of simultaneous ignitionof all the sheets 12. The copper plate 42 was placed on a flat surfacewith the layer of tin-silver solder 32B facing upwards. The assembly 24was positioned on top of the copper plate 42 so that it completelycovered the bonding region 26. Each boron carbide tile 40A-40F wasplaced above the assembly 24 in the desired configuration, with thelayers of tin-silver solder 32A facing down, in contact with the sheets12 of the assembly 24. In the present example, the boron carbide tiles40A-40F were positioned end to end, in contact with each other andaligned with the rectangular bonding region 26, as shown in FIG. 16. Acompliant layer 38 consisting of a thin layer of hard rubber, matchingthe configuration of the bonding region 26, was placed above the boroncarbide tiles 40A-40F. An aluminum spacer plate 34 was positioned abovethe compliant layer 38, and aligned with the bonding region 26. Theentire arrangement was transferred to a hydraulic press, and a load of65,250 lbs was applied to the arrangement by the press platens 36A and36B. The RCM sheets 12 were ignited electrically, simultaneously attwelve evenly spaced points corresponding to each of the ignitionbridges 22 and one at each end of the assembly 24, resulting in abonding reaction between the copper plate 42 and the boron carbide tiles40A-40F.

Example 6

In this example, an assembly 24 of RCM sheets 12 was utilized to bondtwo curved component bodies 44A and 44B over matching non-planar(curved) surfaces, as illustrated in FIG. 17. Specifically, a componentbody 44A comprising a curved sheet of steel having a thickness of 0.015inches was bonded to a component body 44B comprising a curved boroncarbide tile. The bonding region was not restricted to a regular shape,and had a surface area of approximately 111 sq. inches. Fusible layers32A and 32B of tin-silver solder were pre-applied to the sheet steel andto the boron carbide tile. Six RCM sheets 12 were pre-assembled into anassembly 24 by taping six reactive multilayer foil ignition bridges 22across gaps G outside of the irregular curved bonding region 26 andpressing two indium solder tabs across gaps G inside of the bondingregion 26. The boron carbide tile was placed on a form-fitting mold 46with the fusible layer 32B of tin-silver solder facing upwards. Theform-fitting mold 46 was lined with a compliant layer 38 of rubber toaccommodate surface imperfections. A free-standing fusible layer 48,consisting of a silver-tin-titanium solder (S-Bond® 220) comprising fourpieces each 3 inches wide, was positioned over the boron carbide tile44B. The RCM assembly 24 was then positioned on top of the free-standingfusible layer 48 so that it completely covered the irregular curvedbonding region 26. The steel sheet 44A was next placed above theassembly 24 with the fusible layer 32A of tin-silver solder facing down,in contact with the RCM sheets 12. A matching form fitting mold 50 waspositioned above the steel sheet 44A, and the entire assembly wastransferred to a hydraulic press. A load of 32,000 lbs was applied tothe assembly by the press platens 36A and 36B, and the reactivecomposite material was ignited electrically, simultaneously at tenpoints evenly spaced around the peripheral edges of the bonding region26 to initiate the bonding reaction throughout the curved irregularbonding region.

The present disclosure provides a novel method of bonding metal orceramic tiles or plates to metal backing plates for use as targets inphysical vapor deposition processes (sputtering). In one embodiment, themethod utilizes reactive multilayer foil to heat the interface betweenthe plates above the melting point of solder or braze layers pre-appliedto the plates, allowing the solder or braze to fuse together and to thefoil and join the plates (FIG. 18). This method permits soldering orbrazing of materials with large differences in coefficient of thermalexpansion due to minimal heating of the plates. Targets and backingplates bonded in this manner exhibit much lower deflection and residualstress than targets and backing plates bonded by reflow. The finemicrostructures obtained in the re-solidified solder or braze due torapid cooling after joining exhibit higher strengths than do solders orbrazes after reflow.

Targets bonded with this invention may be operated at powers 30 to 100%higher than targets bonded with elastomers or indium solder withoutcracking or separating from their backing plates. Targets bonded withthis invention may also have superior bond line uniformity and thus maysputter more uniformly than targets bonded by other means.

Target materials that may be bonded in this manner include but are notlimited to aluminum oxide, quartz, indium tin oxide, boron carbide,silicon carbide, silica glass, silicon, graphite, CVD diamond, aluminumnitride, zinc oxide, lanthanum manganese oxide, other oxides, othercarbides, and other nitrides. Metals for targets or backing platesinclude but are not limited to lanthanum, zirconium, nickel, cobalt,tungsten, titanium, copper, brass, aluminum, titanium-tungsten alloys,copper-tungsten alloys, InCuSil® and other braze alloys. Solders thatmay be used to join the targets and backing plates include but are notlimited to PbSn, SnAg, SnZn, and SAC.

The following case studies demonstrate the bonding with reactivemultilayer foil of various sputtering target materials to backing platematerials. Residual stress analysis by finite element modeling (FEM)compares conventional bonding to bonding with reactive multilayer foil.Measured bond strength data is presented and bond quality is discussed.Furthermore, a side by side performance comparison between aconventionally bonded ceramic (B₄C) target, using indium solder, and aceramic (B₄C) bonded target using reactive multilayer foils and highermelting temperature SnAg solder is presented. The B₄C target bonded withreactive multilayer foil was run at double the power without anycracking or debonding from the backing plate compared to theconventionally bonded B₄C target for a significantly longer duration.

Residual Stress Analysis

Finite Element Modeling (FEM) of the bonding of a ceramic target, B₄C,to a metal backing plate, CuCr, was performed. The geometry consisted ofa 6″×6″×0.25″ B₄C target bonded with 96.5Sn3.5Ag solder to a 6″×6″×0.31″Cu—Cr plate. Two separate cases were analyzed. The first case was aconventional bonding operation where the entire assembly was heateduniformly above the melting temperature of the solder and then cooleduniformly with a bond forming once the solder solidified (below 221°C.). The second case was a bonding operation using reactive multilayerfoil as a heat source with non-uniform heating and cooling of the solderand the components. A cross-sectional temperature profile captured atthe moment of solder solidification (FIG. 19) was first generated byindependent finite difference modeling and used as an input for the FEManalysis. The residual stress, expressed as the von Mises stress, afterboth these bonding operations is represented in FIG. 20. The residualstresses in the components and at the bond line are about an order ofmagnitude lower for the bonding operation using reactive multilayer foilcompared to the conventional bonding operation. In fact, the predictedresidual stresses for the conventional bonding operation suggest thatthe bonding would not be possible, as is found in practice.

Bond Strength

The bond strengths of various configurations joined with reactivemultilayer foil have been measured. Table 1 lists some shear strengthswe have measured for bonding with different solders. The measuredstrengths are found to depend on the strength of the solder used and noton the combination of materials bonded. Hence bonds formed with indiumsolder are limited in strength by the strength of indium to 4-6 MPa(580-870 psi), while bonds formed with SnAg measure 23-28 MPa (3335-4060psi) due to the higher strength of SnAg solder. Of further note is thefact that where it is possible to form conventional reflow bonds whenthe CTE mismatch between sputter target and backing plate is notsignificant, the measured strengths are generally about 10% lower thanthe bonds formed with reactive multilayer foil. This higher strength canbe attributed to the refined microstructure formed due to the rapidcooling during bonding with reactive multilayer foil.

TABLE 1 Measured shear strength of bonds formed using reactivemultilayer foil for different solder alloys: Reactive multilayer foilConventional reflow Solder bonds (MPa) bonds (MPa) In 4-6 2-3 SnPb 17-20SnAg 23-28

Bond Quality

The quality of large area sputtering target to backing plate bonds, upto 300 sq. inches, using reactive multilayer foil has been found to beconsistently very good and beyond the capability of current commercialprocess. For any combination of sputter target, backing plate andsolder, the required thickness and properties of the multilayer foil canbe chosen by running custom written finite difference software thataccounts for thermal transfer. This ensures that sufficient heat istransferred into the solder for melting, while not heating up thesputter target and backing plate. FIG. 5 shows a cross section of a bondbetween two brass discs (8 in diameter) achieved by melting 63Sn37Pbsolder with reactive multilayer foil that is 60 μm (0.0024 in) thick.For this bond the 63Sn37Pb solder layers had been pre-applied to thecomponents with thicknesses of approximately 150 μm (0.006 in). Theamount of solder that was melted by the reactive multilayer foil duringbonding is indicated by the layer of refined microstructure of thesolder adjacent to the multilayer foil. Hence, about half the thicknessof the pre-applied solder layers was melted during bonding. FIG. 5 alsoindicates good wetting between the reactive multilayer foil and thesolder with no voids observable. Furthermore, it is apparent that thereactive multilayer foil does not form a continuous layer, but rathercracks during bonding with the cracks filled in by the molten solder.This results in a reinforced composite material containing hard longparticulates, the intermetallic product of the reactive multilayer foil,in a ductile matrix, the solder.

The percentage bond coverage of sputter targets, including ceramictargets, bonded to backing plates using reactive multilayer foilsexceeds the standard industry requirements of total coverage >95%, nosingle void >2% and no edge voids. The typical coverage for reactivemultilayer foil bonds is greater than 98%. FIG. 22 (or 15) shows anultrasonic scan of the bond line of a 12 in×12 in titanium alloy plate(CTE=8.6 μm/m/° C.) bonded by melting 96.5Sn3.5Ag with reactivemultilayer foil to an aluminum backing plate (CTE=23.6 μm/m/° C.). Thebond coverage is measured to be >99% without any edge voids, thusexceeding the current industry standard. Various dark lines can beobserved in the scan. The non-straight dark lines are caused by cracksin the reactive multilayer foil that are filled in with solder, similarto that shown in FIG. 21. The straight dark lines indicate that multiplepieces of reactive multilayer foil were used to achieve completecoverage of the bond area.

Case Study

Boron carbide (B₄C) sputtering targets were bonded to copper-chromiumalloy backing plates by a conventional reflow solder process usingindium and by a multilayer reactive foil approach using 96.5Sn3.5Ag. Inboth cases the bonded target was a 4 piece construction of 0.25 in thickB₄C tiles with 90 degree butt joints bonded to a single backing plate.Each B₄C tile measured 6.25 in long and 6 in wide so that the total bondarea was 25 in long and 6 in wide.

The two B₄C targets were evaluated by DC magnetron sputtering inidentical cathodes in the same vacuum chamber. All sputteringparameters, except for power input, were also identical. Theconventionally bonded target was run at 2 kW, while the target bondedwith reactive multilayer foil was run at 4 kW. The conventionally bondedtarget cracked after the first use, running for less than 10 hours.These cracks can be seen in FIG. 23. In addition, debonding of one ofthe B₄C tiles from the backing plate was observed after further use. Thetarget that was bonded using reactive multilayer foil was run at twicethe power in multiple uses in excess of 200 hours with no evidence ofcracking and no evidence of debonding. The significantly betterperformance of the reactively bonded target at higher powers can beattributed to two main factors. First, the bonding operation usingreactive multilayer foil imparted very little residual stress on thebond and the components and hence lowered the driving force for crackingduring use. The second reason was due to the fact that a higher meltingtemperature solder was used. The 96.5Sn3.5Ag solder melts at 221° C.compared to indium solder that melts at 157° C. This means that the bondcan tolerate higher temperatures generated at higher input powers. It isthe bonding with reactive multilayer foils that enables the use of96.5Sn3.5Ag solder.

ITO Case Study

Three identical Indium Tin Oxide (ITO) sputtering targets (7.6 cmdiameter) were bonded to copper backing plates using three differentbonding processes:

(1) Conventional reflow of InSn solder

(2) Elastomer bonding

(3) NanoBond® using NanoFoil® as a local heat source to melt a SnAg typesolder

The three bonded ITO targets were then run sequentially in the samemagnetron cathode under DC power. The power was ramped up in 100 Wincrements, holding for a minimum of 1 hour at each power setting toobserve stable sputtering performance. A summary of each target'sperformance is given in Table 2 below. The target bonded with InSnsolder using a conventional reflow process failed while ramping from 200W to 300 W, when the InSn solder melted and dripped out of the bond,thereby shorting to the anode. Thus the maximum sustainable powerrecorded for this target was 200 W. The target bonded with elastomerstarted to exhibit small cracks when the power was ramped up from 200 Wto 300 W, but seemed to remain stable operating at 300 W. However, whenthe power was ramped from 300 W to 400 W, the cracks became larger (seeFIG. 24) and current and power readings did not stabilize. Eventually,pieces of the target fell off from the backing plate. The maximumsustainable power recorded for this target was thus 300 W, but it mustbe noted that the target had already cracked at this stage. The targetbonded by NanoBond® achieved the highest sustainable sputtering power.Conditions were stable at 400 W. At 500 W the SnAg solder melted andcaused a short. In addition to a solder bond with good thermalconductivity and strength, the NanoBond® also has the advantage of usinga high melting temperature solder, SnAg (Tm=220° C.).

TABLE 2 Performance Summary of ITO targets Max. Power % without Power atMax. Power Improvement Failure Failure Density (InSn Bond Type (W) (W)(W/cm²) baseline) InSn Con- 200 300 4.4 — ventional Elastomer 300 4006.6 50 NanoBond ® 400 500 8.8 100

Alumina Case Study

Two identical alumina (Al₂O₃) sputtering targets (7.6 cm diameter) werebonded to copper backing plates using two different bonding processes:

(1) Elastomer bonding

(2) NanoBond® using NanoFoil® as a local heat source to melt a SnAg typesolder

The two bonded alumina targets were then run sequentially in the samemagnetron cathode under RF power. The power was ramped up in 100 Wincrements, holding for a minimum of 1 hour at each power setting toobserve stable sputtering performance. A summary of each target'sperformance is given in Table 3 below. The target bonded with elastomerstarted to crack at 300 W, but seemed to remain stable at this power.However, when the power was ramped to 400 W pieces of the target felloff from the backing plate (see FIG. 25). The target bonded by NanoBond®performed better and was very stable at 400 W.

TABLE 3 Performance Summary of alumina targets Max. Power % withoutPower at Max. Power Improvement Failure Failure Density (Elastomer BondType (W) (W) (W/cm²) baseline) Elastomer 300 400 6.6 — NanoBond ® 400Not run 8.8 (at >33 to failure least)

Boron Carbide Case Study

Two identical boron carbide (B₄C) sputtering targets (63 cm×15 cm),consisting of four tile pieces, were bonded to copper backing platesusing two different bonding processes:

(1) Conventional Reflow of ln Solder

(2) NanoBond® using NanoFoil® as a local heat source to melt a SnAg typesolder

The two bonded boron carbide targets were run sequentially in the samemagnetron cathode at DC power under production conditions. The targetbonded with conventional reflow of ln solder was run at 2000 W. Asummary of each target's performance is given in Table 4 below. Afterless than 10 hours of use, cracks appeared in the boron carbide as shownin FIG. 26 and after about 100 hours one of the boron carbide tilesdebonded from the backing plate. The target bonded with NanoBond® wasrun at 4000 W. After 200 hours at 4000 W no cracks developed and theboron carbide tiles remained well bonded to the backing plate.

TABLE 4 Performance Summary of Boron Carbide Targets Max. Power Power atMax. Power Sputtering without Failure Density Rate Bond Type Failure (W)(W) (W/cm²) (μm/hr) Conventional 2000 2000 2 1.1 In NanoBond ® 4000 Notrun 4 (at least) 2.3 to failure

CONCLUSION

We have demonstrated a room temperature process of solder bondingsputtering targets to backing plates with no restrictions on thedifferences in CTE of the two materials. This is achieved by localizedheating of solder layers by reactive multilayer foil that releasessufficient heat for melting the solder but not enough to heat up thesputtering target and backing plate. Bonding of ceramic targets to metalbacking plates using solders with high melting temperatures can thus beachieved. High strength bonds are obtainable, limited only by thestrength of the solder chosen. Bond quality is very good with bondcoverage typically >98%. The net result of these high quality, strongmetallic bonds, with high melting temperature solders and with goodthermal and electrical conduction, is that the end user is provided witha durable bonded sputtering target that will not crack during use andcan be run at significantly higher input powers that will result invastly higher deposition rates.

The use of a NanoBond® sputtering target can lead to a 30-100% increaseof sputtering rate (compared to a conventional ln solder reflow bond orelastomer bond). This can consequently lead to significant increases inproduction efficiency. Since the equipment costs in many sputteringproduction processes are very high, this translates to big costreductions per production cycle due to a lowering of overhead costs(especially capital equipment depreciation). For a typical webcoatingprocess, an increase in throughput of 25% could be achieved by halvingthe time spent on the ceramic coating part of the process. The overheadcosts per production run would be reduced by a similar amount. A simplecost analysis shows that the savings can amount to $150,000-500,000 peryear per sputtering system.

The present disclosure can be embodied in-part in the form ofcomputer-implemented processes and apparatuses for practicing thoseprocesses. The present disclosure can also be embodied in-part in theform of computer program code containing instructions embodied intangible media, such as floppy diskettes, CD-ROMs, hard drives, or another computer readable storage medium, wherein, when the computerprogram code is loaded into, and executed by, an electronic device suchas a computer, micro-processor or logic circuit, the device becomes anapparatus for practicing the present disclosure.

The present disclosure can also be embodied in-part in the form ofcomputer program code, for example, whether stored in a storage medium,loaded into and/or executed by a computer, or transmitted over sometransmission medium, such as over electrical wiring or cabling, throughfiber optics, or via electromagnetic radiation, wherein, when thecomputer program code is loaded into and executed by a computer, thecomputer becomes an apparatus for practicing the present disclosure.When implemented in a general-purpose microprocessor, the computerprogram code segments configure the microprocessor to create specificlogic circuits.

As various changes could be made in the above constructions andprocedures without departing from the scope of the invention, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

1. A method of forming a sputter target assembly, comprising the stepsof: providing a backing plate having a top surface, and pre-wetting thetop surface with a solder layer; providing a sputter target having abottom surface, and pre-wetting the bottom surface with a solder layer;introducing a bonding foil, between the backing plate and the sputtertarget, wherein the bonding foil is an ignitable heterogeneousstratified structure for the propagation of an exothermic reaction;pressing the backing plate and the sputter target together and ignitingthe bonding foil, therebetween to melt and bond the solder layer on thebacking plate with the solder layer on the sputter target withoutaffecting the microstructure or flatness of the sputter target in theformation of the sputter target assembly.
 2. The method of forming asputter target assembly of claim 1, further comprising, aligning thetarget and backing plate in a press, prior to pressing and thereafterapplying a load in excess of 50,000 lbs.
 3. The method of forming asputter target assembly of claim 1, wherein the backing plate and thesputter target are made of different materials.
 4. The method of forminga sputter target assembly of claim 1, wherein the solder layers comprisetin in an amount of 63 weight percent and the balance is lead.
 5. Themethod of forming a sputter target assembly of claim 1, wherein thesolder layers are of substantially the same thickness.
 6. The method offorming a sputter target assembly of claim 1, further comprisingelectrically igniting the foil to initiate the exothermic reaction. 7.The method of forming a sputter target assembly of claim 1, wherein thethickness of the bonding foil falls within the range from about 0.002 toabout 0.003 inches.
 8. The method of forming a sputter target assemblyof claim 1, wherein the solder layers have a thickness falling withinthe range from about 0.005 to 0.010 inches.
 9. The method of forming asputter target assembly of claim 1, wherein the pressure is applied viaa hydraulic press, screw or manual press.
 10. The method of forming asputter target assembly of claim 1, wherein the bonding foil is selectedfrom among silicides, aluminides, borides, carbides, thermite reactingcompounds, alloys, metallic glasses and composites.
 11. The method offorming a sputter target assembly of claim 1, the solder material isselected from among indium, tin-lead, or tin-silver.
 12. The method offorming a sputter target assembly of claim 1, wherein the sputter targetis a substantially circular disc comprised of nickel or cobalt.
 13. Asputter target assembly comprising: a sputter target, a backing plateand a bonding foil, disposed between the backing plate and the sputtertarget, wherein the bonding foil is an ignitable heterogeneousstratified structure for the propagation of an exothermic reaction inorder to bond the sputter target to the backing plate without affectingthe microstructure or the flatness of the sputter target in theformation of the sputter target assembly.
 14. The sputter targetassembly of claim 13, further comprising a first solder bond layerdisposed between the backing plate and the bonding foil and a secondsolder bond layer between the target and the bonding foil.
 15. Thesputter target assembly of claim 14, wherein the thickness of thebonding foil falls within the range from about 0.002 to about 0.003inches.
 16. The sputter target assembly of claim 15, further comprisingsolder layers having a thickness that falls within the range from about0.005 to 0.010 inches.
 17. The sputter target assembly of claim 14,wherein the bonding foil is selected from among silicides, aluminides,borides, carbides, thermite reacting compounds, alloys, metallic glassesand composites.
 18. The sputter target assembly of claim 14, wherein thesputter target is a substantially circular disc comprising nickel orcobalt.
 19. A method of forming a bonded plate assembly, comprising thesteps of: providing a first plate having a top surface, and pre-wettingthe top surface with a solder layer; providing a second plate having abottom surface, and pre-wetting the bottom surface with a solder layer;introducing a bonding foil, between the first plate and the secondwherein the bonding foil is an ignitable heterogeneous stratifiedstructure for the propagation of an exothermic reaction; pressing thefirst plate and the second plate together and igniting the bonding foil,therebetween to melt and bond the solder layer on the first plate withthe solder layer on the second plate without affecting themicrostructure or flatness of the second plate in the formation of thebonded plate assembly.
 20. The method of forming the bonded plateassembly of claim 19, further comprising, aligning the second plate andthe first plate in a press, prior to pressing and thereafter applying aload in excess of at least 50,000 lbs.
 21. The method of forming thebonded plate assembly of claim 19, wherein the first plate and thesecond plate are made of different materials.
 22. The method of formingthe bonded plate assembly of claim 19, wherein the solder layers aretin-lead solder.
 23. The method of forming the bonded plate assembly ofclaim 19, wherein the solder layers are of substantially the samethickness.
 24. The method of forming the bonded plate assembly of claim19, further comprising electrically igniting the foil to initiate theexothermic reaction.
 25. The method of forming the bonded plate assemblyof claim 19, wherein the thickness of the bonding foil falls within therange from about 0.002 to about 0.003 inches.
 26. The method of formingthe bonded plate assembly of claim 19, wherein the solder layers have athickness that falls within the range from about 0.005 to 0.010 inches.27. The method of forming the bonded plate assembly of claim 19, whereinthe pressure is applied via a hydraulic press, screw or manual press.28. The method of forming the bonded plate assembly of claim 19, whereinthe bonding foil is selected from among silicides, aluminides, borides,carbides, thermite reacting compounds, alloys, metallic glasses andcomposites.
 29. The method of forming the bonded plate assembly of claim19, wherein the solder material is selected from among indium-tin,tin-lead, or tin-silver-copper.
 30. The method of forming the bondedassembly of claim 19, wherein the second plate is a substantiallycircular, disc-shaped nickel plate.
 31. A large area bonded plateassembly comprising: a first plate, a second plate and a bonding foil,disposed between the first plate and the second plate, wherein thebonding foil is an ignitable heterogeneous stratified structure for thepropagation of an exothermic reaction in order to bond the first plateto the second plate without affecting the microstructure or the flatnessof the second plate in the formation of the bonded plate assembly. 32.The bonded plate assembly of claim 31, further comprising a first solderbond layer disposed between the first plate and the bonding foil and asecond bond layer between the second plate and the bonding foil.
 33. Thebonded plate assembly of claim 31, wherein the thickness of the bondingfoil falls within the range from about 0.002 to about 0.003 inches. 34.The bonded plate assembly of claim 15, further comprising solder layershaving a thickness within the range from about 0.005 to 0.010 inches.35. The bonded plate assembly of claim 31, wherein the bonding foil isselected from among silicides, aluminides, borides, carbides, thermitereacting compounds, alloys, metallic glasses and composites.
 36. Thebonded plate assembly of claim 31, wherein the second plate is asubstantially circular, disc-shaped nickel plate.